X-ray reflectometry (XRR) is a well-known technique for measuring the thickness, density and surface quality of thin film layers deposited on a substrate. Such measurements are particularly useful in evaluating layers deposited on semiconductor wafer substrates in the course of integrated circuit manufacture.
X-ray reflectometers are sold by a number of companies, among them Technos (Osaka, Japan), Siemens (Munich, Germany) and Bede Scientific Instrument (Durham, UK). Such reflectometers typically operate by irradiating a sample with a beam of X-rays at grazing incidence, i.e., at a small angle relative to the surface of the sample, near the total external reflection angle of the sample material. Measurement of X-ray intensity reflected from the sample as a function of angle gives a pattern of interference fringes, which is analyzed to determine the properties of the film layers responsible for creating the fringe pattern. The X-ray intensity measurements are commonly made using a detector mounted on a goniometer. More recently, fast X-ray reflectometers have been developed using position-sensitive detectors, such as a proportional counter or an array detector, typically a photodiode array or charge-coupled device (CCD).
For example, U.S. Pat. No. 5,619,548, to Koppel, whose disclosure is incorporated herein by reference, describes an X-ray thickness gauge based on reflectometric measurement. A curved, reflective X-ray monochromator is used to focus X-rays onto the surface of a sample. A position- sensitive detector, such as a photodiode detector array, senses the X-rays reflected from the surface and produces an intensity signal as a function of reflection angle. The angle-dependent signal is analyzed to determine properties of the structure of a thin film layer on the sample, including thickness, density and surface roughness.
U.S. Pat. No. 5,923,720, to Barton et al., whose disclosure is incorporated herein by reference, also describes an X-ray spectrometer based on a curved crystal monochromator. The monochromator has the shape of a tapered logarithmic spiral, which is described as achieving a finer focal spot on a sample surface than prior art monochromators. X-rays reflected or diffracted from the sample surface are received by a position-sensitive detector.
U.S. Pat. No. 5,740,226, to Komiya et al., describes a method for analyzing X-ray reflectometric data to determine film thickness. After measuring X-ray reflectance as a function of angle, an average reflectance curve is fitted to the fringe spectrum. The average curve is based on a formula that expresses attenuation, background and surface roughness of the film. The fitted average reflectance curve is then used in extracting the oscillatory component of the fringe spectrum. This component is Fourier transformed to find the film thickness.
In order to obtain accurate measurements of film thickness, it is necessary to precisely calibrate the angular scale of the reflection. Such a calibration requires, inter alia, exact control of the zero angle of reflection, so that the angle of the reflected beam relative to the surface can be determined accurately. (In the context of the present patent application and in the claims, the term “zero angle” refers to the orientation of a tangent to the reflecting surface at the point of incidence of the radiation.) To make reflectometric measurements with optimal accuracy, the zero angle at the measurement point should be known to within 0.005°.
Although semiconductor wafers appear to be flat, in practice wafers typically deform slightly when held by a vacuum chuck during production or inspection. The deformation is due both to the vacuum force exerted by the chuck and to the weight of the wafer itself. Furthermore, the chuck may have imperfections, such as a slight bend in its axis, that cause deviations in the zero angle of the wafer as it rotates. As a result, inclination of the surface at different sample points on the surface of a wafer may vary by as much as 0.1-0.2°. Therefore, to perform accurate reflectometric measurements at a well-defined measurement point, it becomes necessary to recalibrate the zero angle at each point that is tested on the wafer surface.